Geoscience Reference
In-Depth Information
Morphotectonic landscapes in Britain
APPLICATIONS
Rocks carry signatures of their formative environment and subsequent history, themes developed in
Chapters 12
and
23,
but our interest is heightened when we can fit them into sequences of real events on the world stage.
Tectonic episodes from Britain's past, comparable with modern plate boundary and ocean basin activity, can be
detected if we know how and where to look. Terranes from Britain's youngest Proterozoic orogen and two younger
Phanerozoic orogens appear in the satellite image of Wales (
Plate 10.7
).
They tell the story of a micro-continental
plate, Avalonia, pirouetting equatorwards from the southern hemisphere continent Gondwanato northern hemisphere
continents of Laurussia c. 620-290 Ma ago. The rock sequences and terranes provide microcosms of the break-up
of one supercontinent (Vendia) and the formation of another (Pangaea). During that time, first the ancient Iapetus
and then Rheic Oceans closed, their resultant intercontinental collisions forming the global-scale Caledonian and
Variscan cordilleran mountain systems respectively (
Figure 10.21
).
The story starts 620-540 Myr ago with accretion of Late Neoproterozoic marginal orogens to Avalonia at approximately
60
o
S on the coast of west Gondwanaland, part of the south polar-centred Vendian supercontinent. The Andes provide
a modern analogue (see this chapter and
Chapter 24),
with subduction creating volcanic island arcs, back-arc basins
and accretionary prisms. Extensive lateral strike-slipfaulting followed as the arc-continent moved obliquely onshore,
forming major collision tectonic lineaments and basins. Fragments of two terranes appear today on the margins of
Wales as inliers, from which younger rocks have been stripped away to provide windows on Britain's ancient basement
crust. The north-western Monian-Rosslare terrane
(
Figure 10.22a
),
outcropping across Anglesey and western Ll yn
(and south-east Ireland) contains fault-bound slivers of gneiss, trench metasediments and metamorphic schists from
the accretionary prism. The generally younger, southern and eastern Avalon terranecontains volcanoclastic, continental
margin and continental sediments and outcrops in Pembroke, south central Wales and the Welsh borderland.
The Welsh basin, now forming the greater part of Wales, separates both elements of the Avalon superterrane. Crustal
subsidence, rising sea levels and marine sedimentation occurred on Avalonia's northern flank, bordering the Iapetus
Ocean, during the Cambrian period. Avalonia began to rift away from Gondwanaland during the early Ordovician period
(485 Ma), progressively closing the Iapetus Ocean to the north and opening the Rheic Ocean to the south. Active
subduction developed as Avalonia converged on Laurentia. The Welsh basin became a back-arc basin with further,
often deep-water, marine sedimentation (
10 km thick) and a short, intense period of late Ordovician volcanic activity
c. 450 Ma (see, pp 262 and
Figure 12.7
).
Most volcanic lavas and volcanoclastic sediments were deposited in
surrounding submarine environments. Thereafter the Welsh basin continued to subside before gradually shelving as
the Iapetus Ocean finally closed. Deep-water turbidites accumulated during the Silurian period (435-405 Ma), giving
way eventually to shallow shelf carbonates in the Welsh borderland
(
Figure 10.22a
).
Major crustal shortening and
uplift accompanied the resultant intercontinental collision, suturing south-east and north-west 'Britain' together along
a north-east-south-west line during the Caledonian Orogeny
(
Figure 10.22b
)
The final piece of the Welsh tectonic mosaic was created from erosion products of this and successive land-masses
during the Devonian and Carboniferous periods (405-290 Ma), complicated by periodic crustal extension, fault-bound
subsidence and marine transgression associated with gradual closure of the Rheic Ocean to the south. Two distinct
terranes were formed. Devonian Old Red Sandstones are largely terrestrial, alluvial fan and floodplain accumulations
in desert conditions about 20
o
south of the equator. They contrast with primarily shallow marine sequences of shelf
limestones, deltaic sandstones and then coal measures formed as our evolving crust crossed the equator. Rheic
Ocean closure by 290 Ma, and the accompanying Variscan orogeny, occurred as part of the great clustering of
continents which formed Pangaea, uniting Gondwanaland and Laurentia. The suture lies across south central Europe
but its tectonic effects created a Variscan thrust front across southern Britain, including south Wales.
Relatively little is known of Welsh crustal evolution between that time and the Cenozoic era, other than it probably
shared in general erosion, subsidence and burial by largely marine Mesozoic (Jurassic and Cretaceous) sediments
across southern Britain as Pangaea rifted apart and the Atlantic Ocean began to open. It shared passive European
continental margin conditions as the Atlantic Ocean widened during the Palaeogene period (early Cenozoic, between
65 Ma and 23 Ma). The Welsh landform of the satellite image began to take on its modern shape during the Neogene
